Method and composition for treating aluminum and aluminum alloys



2,883,311 lc Patented AIM- 1959 METHOD AND COMPOSITION FOR TREATINGALUMINUM AND ALUMINUM ALLOYS David Halpert, Philadelphia, Pa., assignorto Vertol Aircraft Corporation, Morton, Pa., a corporation ofPennsylvania No Drawing. Application October 1, 1956 Serial No. 614,779

6 Claims. (Cl. 134-3) This invention relates to the surface treatment ofaluminum and aluminum base alloys for the removal of oxide films, andmore particularly to the composition and method for treating aluminumsurfaces so as to maintain a low electrical resistance for a substantialperiod of time.

Surface cleaning of aluminum and it alloys, for producing andmaintaining a smut-free bright surface, is made diflicult by the rapidformation of oxide coatings on aluminum when exposed to the atmosphere.It is essential that the deoxidizing solution dissolve away the naturaloxide film without attacking the aluminum metal too sharply, which wouldresult in etching or pitting of the surface. This can be done if auniform reaction takes place between the aluminum and the deoxidizingsolution such that the solution acts on all parts of the surfacesimultaneously and with the same reaction speed.

It is therefore, a primary object of this invention to provide achemical deoxidizing solution for removing the oxide coatings onaluminum and its alloys wherein the composition agents are readilycontrollable at the room operating temperature of the solution, andfurther, wherein the dimensional tolerance losses of the aluminum arepositively controlled by the use of an inhibitor.

A further object of the invention is to provide an aluminum deoxidizingsolution comprising an aqueous acid solution containing fluorides,chromates or dichromates and nitric acid in certain well definedproportions and ranges of concentration for producing an electricallylow resistant surface without excessively removing the aluminum basemetal.

Further objects and advantages of the chemical deoxidizing solution willbecome apparent from the follow ing description.

The term deoxidizing solution is used throughout the description inplace of chemical cleaning solution because it more accurately definesthe function of the solution, which is to remove the characteristicoxide coating on aluminum and its alloys.

In general, the composition of the deoxidizing solution consists ofrelatively high concentrations of nitric acid and fluorides incombination with hexavalent chromium ions. Nitric acid (70% concentrate)is preferably used in an amount within the range of from about 80 to 90cc. per liter. The fluoride constituent may be introduced in the form ofa simple or complex fluoride which is soluble in nitric acid withoutproducing objectionable precipitates with the chromium ions or thealuminum ions that may find their way into the solution by metaldissolution. Sodium fluoride is preferably used for supplying thefluoride ion since it is easily handled and relatively eco nomical. Ithas been found that the fluoride ion concentration of the deoxidizingsolution should be maintained at an amount approximately equivalent tothat obtained with l to 45 grams of sodium fluoride per liter ofsolution. Also, by varying the amount of sodium fluoride introduced intosolution, it is possible to control the solution pH value, which forbest results should be maintained within a range of 0.2 to 1.7, thepreferred pH range being 0.35 to 0.9.

Chromic acid is a preferred constituent for supplying the hexavalentchromium ion which acts as the inhibitor for the otherwise vigorouslyreacting mixture of fluorides and nitric acid. The hexavalent chromiumion when properly proportioned with the fluoride ion and the nitricacid, tends to regulate the dissolving action of the deoxidizingsolution on the aluminum such that its dimensional losses do not exceed0.0002" even for an immersion time of approximately 5 minutes in a 60 to95 F. solution. It was found that the chromium ion concentration shouldbe maintained in an amount approximately equivalent to that obtainedwith 1 to 400 grams of anhydrous chromic acid per liter of solution.

Thus, the deoxidizing solution of the present invention essentiallycomprises:

Proper Constituent tlon Preferred Range Nitric Acid-BNO; (70%concentrate). -cc./l. 80 to 980 330 Sodium Fluoride-Nah (anhydrous) g./l1 to 45 15 Chromic Acid-CrO; (anhydrous).. /l. 1 to 400 l5 2 WaterH 0Balance Balance Prior to the immersion of the aluminum or its alloy inthe deoxidizing solution, the surface is preferably cleaned of heavyoil, grease, buffing compounds and dirt by a suitable cleaner such as avapor degreaser containing trichlorethylene or perchlorethylene. Forfurther removal of any light residual films remaining after vapordegreasing, a mild alkaline cleaner may be used. Though a number ofcommercial cleaners of this type are available, a suitable cleaner ofthis general type can be prepared by using 4 ounces of sodium carbonateand 4 ounces of sodium phosphate per gallon of water. This solution isgenerally used at a temperature of to F. for about 1 to 3 minutes. Itshould be noted however, that mild alkaline cleaners which functionwithout etching, are not efiective in removing the oxide coating onaluminum surfaces and for this purpose the surface to be cleaned is thensubjected to a further treatment in the deoxidizing solution of thisinvention.

When it is desired to prepare the surface for the application ofprotective or decorative coatings such as those obtained by anodizing orchromatizing or to prepare the surface for the plating thereon ofmetallic coatings, the aluminum or aluminum alloy piece is immersed inthe deoxidizing solution for a period of anywhere from 5 seconds to 5minutes depending on the thickness of the surface film and previoustreatment. The surface thus produced is rather bright and uniform inappearance and not pitted.

The deoxidizing solution of the present invention has been foundextremely useful in preparing aluminum and its alloys for spot welding.In this connection, it should be noted that successful spot welding ofaluminum depends upon obtaining a low electrical surface resistance,usually less than 100 microhms for the mating surfaces, and maintainingthis low resistance until such time as the parts are welded. In order todetermine the immersion time required to produce a low electricalsurface resistance, test strips of aluminum and several of its alloyswere immersed in the hereinabove set forth preferred deoxidizingsolution at temperatures ranging from 60 to 95 F. and for periods oftime ranging from 2 seconds to 5 minutes, after which the strips wereremoved from the solution, thoroughly rinsed in water and dried. Thesurface resistanceof these test strips was then measured by means of anelectrical resistance measuring device of the Kelvin Bridge type, and byplotting resistance readings against time of immersion, it was notedthat a rapid 5 decrease in surface resistance occurs and that a minimumsurface resistance is obtained for an immersion time of approximately 5to seconds. The plotted curve also indicated that for longer immersiontimes, the surface i resistance increases and becomes greater than 100microhms for immersion times greater than approximately seconds.However, by varying the proportions of the constituents within thedisclosed ranges, deoxidizing solutions were obtained by which thesurface resistance remained below 100 microhms for immersion periodssubstantially in excess of 20 seconds.

It was found that the lower the resistance obtained as the result of a 5to 10 second immersion, the more stable the deoxidized surface was inmaintaining its low resistj ance. For example, the immersion time whichproduced a low resistance surface having the greatest stability or iso-called shelf life for 24S clad aluminum, was determined l to be 10seconds. This low surface resistance, of less than 10 microhms, wasmaintained for approximately 2 days and after 30 days was only 30microhms.

It was also found that electrically low resistance surfaces could beobtained and maintained for prolonged periods by using the hereinaboveset forth deoxidizing solution with small amounts of copper dissolvedtherein.

; The copper was intr uce 1n e orm of a so u e sa i such as copper oxidewhich is compatible with the solution such that the anion will not formprecipitates with other solution constituents. The presence of copperions in the deoxidizing solution may be as much as 1' that obtained with1 gram of copper oxide per liter of solution, although the preferredconcentration is that obtained 2 with approximately 0.3 gram per liter.Excellent results were also obtained with the addition of other metalsselected from the group consisting of iron, zinc, magnesium andmanganese which were introduced into the solution in quantities similarto that of the copper and in the form of zinc oxide, magnesium oxide andpotassium permanganate. It is believed that these added metals plate byimmersion on the aluminum and aluminum alloy surfaces to thereby form aprotective film which acts as an oxidation inhibitor without affectingthe surface resistance.

Having thus described my invention, what I claim is:

l. The process of treating aluminum and its alloys which comprisesimmersing the metal in an aqueous solution containing 8 to 98% nitricacid (70% concentrate) and containing fluoride and chromate ions inconcentrations corresponding to that obtained in a solution containing 1to 45 grams of sodium fluoride per liter of solution and 1 to 400 gramsof anhydrous chromic acid per liter of solution for a time suflicient toinitially reduce the surface resistance of the metal to below 100microhms, removing the metal from said aqueous solution before thesurface resistance again reaches 100 microhms and thereafter rinsing thethus treated metal.

2. A process according to claim 1 in which the aqueous solution containscopper ions in concentrations corresponding to that obtained in asolution containing up to 1 gram of copper oxide per liter of solution.

3. A process according to claim 1 in which the aqueous solution containsmetals selected from the group consisting of copper, iron, zinc,magnesuim and manganese.

4. An aqueous deoxidizing solution consisting of 8 to 98% nitric acidconcentrate), fluoride and chromate ions in concentrations correspondingto that obtained in a solution containing 1 to 45 grams of sodiumfluoride per liter of solution and 1 to 400 grams of anhydrous chromicacid per liter of solution, and copper ions in concentrationscorresponding to that obtained in a solution containing up to 1 gram ofcopper oxide per liter of solution.

5. An aqueous deoxidizing solution consisting of 8 to 98% nitric acid(70% concentrate), fluoride and chromate ions in concentrationscorresponding to that obtained in a solution containing 1 to 45 grams ofsodium fluoride per liter of solution and 1 to 400 grams of anhydrouschromic acid per liter of solution, and metal ions selected from thegroup consisting of copper, iron, zinc, magnesium and manganese.

6. The process of treating aluminum and its alloys which comprisesimmersing the metal in an aqueous solution consisting of 8 to 98% nitricacid (70% concentrate) and fluoride and chromate ions in concentrationscorresponding to that obtained in a solution containing 1 to 45 grams ofsodium fluoride per liter of solution and l to 400 grams of anhydrouschromic acid per liter of solution for 5 to 20 seconds, removing themetal from said aqueous solution and thereafter rinsing the thus treatedmetal.

References Cited in the file of this patent UNITED STATES PATENTS2,428,749 De Long Oct. 7, 1947 2,598,889 Caugherty June 3, 19522,705,500 Deer Apr. 5, 1955 FOREIGN PATENTS 726,660 Great Britain Mar.23, 1955

1. THE PROCESS OF TREATING ALUMINUM AND ITS ALLOYS WHICH COMPRISESIMMERSING THE METAL IN AN AQUEOUS SOLUTION CONTAINING 8 TO 98% NITRICACID (70% CONCENTRATE) AND CONTAINING FLUORIDE AND CHROMATE IONS INCONCENTRATIONS CORRESPONDING TO THAT OBTAINED IN A SOLUTION CONTAINING 1TO 45 GRAMS OF SODIUM FLUORIDE PER LITER OF SOLUTION AND 1 TO 400 GRAMSOF ANHYDROUS CHROMIC ACID PER LITER OF SOLUTION FOR A TIME SUFFICIENT TOINITIALLY REDUCE THE SURFACE REISITANCE OF THE METAL TO BELOW 100MICROHMS, REMOVING THE METAL FROM SAID AQUEOUS SOLUTION BEFORE THESURFACE RESISTANCE AGAIN REACHES 100 MICROHMS AND THEREAFTER RINSING THETHUS TREATED METAL.